where the coefficients \(\zeta _n\)  are time–independent.
Creation of a two–particle DM state with momenta \(p,q\)  from the vacuum is described by the amplitude (in Peskin–Schroeder conventions [1]})
\(-i \int _{-\infty }^\infty dt \langle f | V(t) | i \rangle = - i \,{ \lambda _{\phi s} \over 2 } \, (2\pi )^4 \delta ({\bf {p}} + {\bf {q}}) \sum _{n=1}^\infty \zeta _n \delta (E_p +E_q -n \omega ) \;,\) 
